a) Field of the Invention
The invention is directed to an arrangement for fastening permanent magnets to rapidly rotating rotors of electric machines. It is preferably applied in cases of special demands on mechanical strength at high peripheral speeds and high operating temperatures and suppression of eddy currents.
b) Description of the Related Art
Arrangements in which the magnets are fastened to a magnetizable rotor support by means of a glue and are fixed in place by a nonmagnetic wrapping are known in the prior art. This method is limited as far as its possible uses because of the limiting holding power of adhesives at elevated temperatures and large centrifugal forces due to high rotating speeds.
In another solution, the magnets are “buried” in a bundle of laminations. This method can only be applied in certain mechanical and magnetic designs and reduces the magnetic exploitation of the system.
Closed shells (wrappings) are another way to secure magnets to the rotor. Nonmagnetic wrappings of nonmetallic materials such as glass fibers, carbon fibers, various plastics and also of metals, e.g., nonmagnetic steel, aluminum or titanium, are known from the prior art. While wrappings of electrically conductive material usually have the disadvantage that the rotor parts are additionally heated because of the formation of eddy currents, wrappings of plastic fibers are often insufficiently stable (i.e., they exhibit impermissible elongation) at elevated temperatures in spite of the high cost of manufacture.
U.S. Pat. No. 6,603,232 B2 discloses a permanent magnet holding device for high-speed rotors in which the permanent magnets are placed in the outer rotor enclosure in inverted U-shaped retaining parts which are held by pole pieces that are screwed to the rotor enclosure in frictional engagement or connected directly to the rotor in a frictional engagement by screws. This frictionally engaging variant is disadvantageous primarily because of the risk that the screws will loosen by themselves due to vibrations, which requires that a stable outer wrapping is always provided in addition in order to reduce the safety risk.